This invention is directed to switching circuitry which may, for example advantageously be employed for selectively actuating or deactuating the stereo expansion circuitry of an audio system.
Stereo expansion in audio systems is well known in the art and has been available for many years. In such systems, the right and left channel signals are processed in a manner which makes it appear to the listener that the separation of the speakers is much greater than the actual physical separation. Stereo expansion is described in U.S. Pat. No. 4,495,637 of Bruney.
FIG. 1 is a simplified block diagram showing the selection of an expanded stereo system. The right (R) and left (L) audio signals are received on input lines 1 and 1a respectively. A selection switch 2 is shown as a double-pole double-throw switch for purposes of illustration. In an actual stereo receiver, the switch 2 would be an electronic switch. When expanded stereo is not desired, the switch is coupled to lines 1 and 1a and the R and L signals pass directly through the switch 2 to output lines 8 and 9. The input lines 1 and 1a are also coupled to a stereo expansion circuit 3 having output lines 4 and 5. The expansion circuit 3 combines the R and L audio signals for producing the expanded audio signals L+X(L-R) and R+X(R-L) which are available on output lines 4 and 5 of the expansion circuit 3.
FIG. 2 is the schematic diagram of another switching arrangement for selectively providing stereo or expanded stereo signals and will be described in detail below. Briefly, it uses a transmission gate coupled between inputs of operational amplifiers. As shown, the transmission gate may comprise a field effect transistor. Such a switch arrangement is much simpler than switch 2 shown in FIG. 1. However, the present inventor has recognized that depending on the type of FET utilized, the transmission gate itself may introduce signal distortion.